fulltext

DOI 10.1393/ncc/i2011-11102-x

Colloquia_{: IFAE 2011}

IL NUOVO CIMENTO Vol. 34 C, N. 6 _{Novembre-Dicembre 2011}

The performance of the ALICE TOF detector

Dipartimento di Fisica, Universit`a di Bologna and INFN, Sezione di Bologna - Bologna, Italy (ricevuto il 29 Luglio 2011; pubblicato online il 21 Dicembre 2011)

Summary. — The performances of the ALICE Time Of Flight detector are

pre-sented, based on the 2010 data taking. Hardware status, time resolution and PID capability are shown and compared with the design values. First results in Pb-Pb at√sN N= 2.76 TeV are reported.

PACS 29.40.Cs – Gas-filled counters: ionization chambers, proportional, and avalanche counters.

1. – Introduction

ALICE is an experiment at CERN LHC designed and optimized to study Pb-Pb col-lisions with √sN N = 5.5 TeV. The investigation of colored deconfined matter, the so

called Quark Gluon Plasma, is its main goal. To achieve this it has been designed with an excellent tracking system and very high particle identification (PID) performance. Time Of Flight system is one of the main ALICE detectors for particle identification [1]. This detector covers a cylindrical surface of about 150 m2_{, with an inner radius of 3.7 m.} It has full azimuthal acceptance (with a segmentation in 18 sectors) and a polar ac-ceptance of |η| ≤ 0.9. In each sector 91 Multigap Resistive Plate Chamber (MRPC) have been installed. The MRPCs have an active area of 7.4× 120 cm2, with ten 250 μm width gas gaps delimited by 400 μm tick soda-lime glasses which are grouped into two stacks, placed on both side around a central anode. External electrodes are obtained from 550 μm tick glasses with a specially developed acrylic paint loaded with metal oxy-des, giving an average surface resistivity between 2 and 25 M Ω/. The strips work in avalanche mode regime, with E∼ 100 kV/cm. Measurements at test beam for a sample of mass-production MRPCs have shown an efficiency M RP C > 99% and a time

resolu-tion σM RP C < 50 ps, at a working voltage of 13 kV [2]. Without beam operations, on

average, the MRPC strips draw a current less than 1 nA. Finally, each strip is divided into 96 readout channels of about 10 cm2 _{of active area. During 2010 data taking the} system used on average 96% of its 157248 readout channels.

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252 A. AGOSTINELLI on behalf of the ALICE COLLABORATION

Fig. 1. – Top: measured resolution during Pb-Pb data taking. Bottom: mass distribution of particle which reached TOF from a single Pb-Pb collision.

2. – Results from 2010 data taking

First results on identified particle spectra in p-p collisions at 900 GeV have been published in [3]. Here selected results from last year heavy ions runs are shown. The measure of global time resolution (σtot) is reported in fig. 1. For each track, the quantity

tmeasured− texpected is related to the TOF time resolution (σT OF), the time zero of a

single Pb-Pb collision (σt-Zero) and the time resolution due to tracking (σt-T rack). In

Pb-Pb, with σt-Zero and σt-T rack typical values of 10 and 20 ps respectively, σT OF is

about 80 ps, as from design value. These distributions will be improved by single channel time-slewing corrections and time-walk inside a pad. Considering an expected time resolution of 90 ps, TOF allows π/K (and K/p) separation better than 2σ up to about 3.0 GeV/c (4.4 GeV/c). Mass distribution for the particles with a measured time of flight from TOF detector in just a single Pb-Pb event is also shown. Particle identification at an event by event level becomes in this way a straight and amazing reality for the TOF detector.

REFERENCES

[1] ALICE Collaboration, J. Phys. G: Nucl. Part. Phys., 32 (2006) 1295.

[2] CERN/LHCC 2000-012, ALICE TDR, 8 (2000) ALICE TOF Technical Design Report. [3] ALICE Collaboration, Eur. Phys. J. C, 71 (2011) 1655.